In German Patent Application No. 44 27 006 (not prior art) the remaining lifetime of a contactor in the disconnection operation is derived from the time difference between the start of the armature opening movement and the start of contact opening. Using an analysis algorithm, a microprocessor determines from the time difference the momentary value of the contact spring action, which decreases due to erosion from its new value (=100% remaining lifetime) to its minimum (=0% remaining lifetime).
The time signals required to do so are detected by interrupting an auxiliary current path over the armature and yoke of the solenoid actuator and on the basis of the contact voltage at the main contacts, and are converted to defined voltage pulses.
To simplify the contact voltage measurement, it is proposed according to German Patent Application No. 1 96 03 310.1 that contact opening, specifically in a three-phase system, be performed by monitoring the voltage, specifically at an artificial neutral point. This makes it possible to switch the device for determining the remaining lifetime as an independent add on unit in the load circuit between the contactor and the electric consumer, which is connected to the contactor with only one communication line for the opening of the armature-yoke contact.